Released for PlayStation 2, Xbox and PC few years ago, this game was made available for free a while back. Voiced by actor David Duchovny of X-Files the game is overall a semi-decent shooter. Approximate Size: 2GB

This one is a RTS/third-person action hybrid. Unusual but ultimately fun. You get to assume the roles of various historic figures like Alexander the Great, Julius Caesar, and similar. Approximate Size: 2.5GB

This is another title that went from a commercial release (on Xbox, PS2, PC) to freeware. Similar to other horror/survival games like Resident Evil and Silent Hill, this one is completely free with one caveat: it is ad-supported. Approximate Size: 1.25GB

This is a pretty dated old-school adventure game, but fun nonetheless. I actually bought a copy back in the 1990s in CD form which I still have somewhere, although I don't remember finishing it. It was made freeware a while back. Approximate Size: 60MB (full CD edition)

Open source port for Windows (Mac port available as well) of the popular DOS game Tyrian. This game is updated constantly and is extremely fun. Approximate Size: 400KB for the OpenTyrian executable and 4.5MB for the Tyrian 2.1 content.

(Install Instructions: Download the Tyrian 2.1 files and extract them to a folder. Then download the latest OpenTyrian build and extract the content to the previously created folder of Tyrian 2.1)

This is one of my personal favorites. A quality multiplayer first-person shooter game with bot-based single player campaign along with some pretty impressive graphics. This game, when settings are maxed out can bring down even high-end systems to their knees, in Crysis-like fashion. At least that was my experience with the Linux builds. Windows builds might be more optimized. Approximate Size: 900MB

I assume pretty much everyone is familiar with the C&C series. A while back the original C&C Gold was released for free. While the graphics are a bit dated by modern standards, the gameplay and the evolving storyline are outstanding. Approximate Size: 1.1GB

This is the sequel to the original game posted above. Gameplay is solid and storyline about time travel into the past in order to kill Hitler and Stalin taking over the world is pretty interesting (Nazis never happened). It features slightly better graphics than the original. Approximate Size: 1GB (for both discs)

I bought the C&C: The First Decade collection a while back and I've been replaying some of the Red Alert and Red Alert 2 campaigns. It has been so long since I played those games (10+ years?) that I don't remember too much even though I'm pretty sure I finished them. Having to relearn the game dynamics all over again is pretty fun I must admit. Even the DOS-era graphics come across as charming.

A genre that has been largely missing from this thread... adventure games.

A Second Face: The Eye of Geltz is Watching You

While the genre might be antiquated, adventure games are still around. This game was released back in 2008 and features some pretty stunning artwork, something that was always a cornerstone of adventure games. Approximate Size: 100MB

More first-person shooter mayhem, this time from a Korean developer. Sudden Attack is a pretty nice looking online/multiplayer shooter with a large international community. Tons of weapons and overall decent production standards. Approximate Size: 500MB